Remote tripping devices or protection signal transmission devices are used for transmitting protection or switching commands for distance protection in electrical high-voltage and medium-voltage networks and systems. Protection commands result, for example, in a circuit breaker being opened directly or indirectly and, in consequence, in electrical disconnection of a part of the network or of the system. Conversely, other protection commands result in opening of a circuit breaker being prevented. Protection commands must be transmitted, for example, from one section of a high-voltage line to another. To do this, a transmitter in a remote tripping device produces analog signals in accordance with the protection commands, which analog signals are transmitted via a signal link. A receiver in another remote tripping device detects the transmitted signals and determines the corresponding number and nature of the protection commands.
By way of example, the analog signals are in a frequency band between 0.3 and 4 kHz. They are either transmitted directly in this frequency band, or are modulated onto a carrier frequency and are demodulated upstream of the receiver, or are transmitted via a digital channel and are reconstructed upstream of the receiver. In any case, an analog received signal is produced at the receiver, in which the presence of individual signals at a different frequency must be detected.
By way of example, FIG. 1 shows a quiescent signal and a number of command signals in the frequency domain and in the time domain for transmission of command signals A, B, C, which correspond to transmitted protection commands or combinations of protection signals. An amplitude axis in the illustration is annotated Amp, a frequency axis is annotated f, and a time axis is annotated t. The transmitted signals are preferably sinusoidal and are each separated in frequency from one another by, for example, 100 Hz to 300 Hz. In a quiescent situation, that is to say when no protection command need be transmitted, a quiescent signal or guard signal G is transmitted continuously instead of this. When a command occurs between the times t1 and t2, one or more command signals are transmitted, and, by way of example, FIG. 1 shows the transmission of signals at two frequencies in the right-hand coordinate system. The receiver detects the presence or the lack of the command signals and of the quiescent signal G continuously, and produces an alarm signal if the signal quality is inadequate, if the two are received together or if no signal whatsoever is received.
The quiescent signal G is used to improve the safety and/or security in that it indicates that no command signal A, B, C is present. A command signal is regarded as having been received only when the quiescent signal G is no longer detected.
It is thus necessary to detect the presence or the lack of individual periodic signals. In this case, it should be borne in mind that the transmission is in general influenced by disturbances which can be characterized by a signal-to-noise ratio, that is to say a ratio SNR between the signal power and the disturbance power. Depending on the nature of the protection command, detection is subject to different requirements in this case, which can be characterized, inter alia, by the following parameters:
Puc Safety and/or security value, that is to say the probability that a command is received falsely, even though it has not actually been transmitted. A low Puc value corresponds to high transmission safety and/or security.
Pmc Reliability value, that is to say the probability that a command which has been transmitted is not received. A low Pmc value corresponds to high transmission reliability.
Tac maximum actual transmission time. This is dependent on the required reliability and on the signal-to-noise ratio SNR, and will be defined in more detail further below.
All known transmission and detection methods have the common feature that, as the disturbance power increases, that is to say the SNR decreases,                the safety and/or security decrease and then, in some cases, increase once again, that is to say the safety and/or security value Puc increases and then, in some cases, decreases again, and        the reliability decreases continuously, that is to say the reliability value Pmc increases continuously.        
A signal at a specific frequency is detected, for example, by means of numerical correlation of the received sum signal with a comparison signal at the same frequency, or by bandpass filtering of the received sum signal. A preprocessed output value from the correlation or from the bandpass filter is compared with a threshold value. If the preprocessed output value is greater than the threshold value, then the signal is regarded as having been detected. The major parameters for the detection process are thus a correlation duration of a correlator or an inverse of the bandwidth of a bandpass filter which are referred to in combined form in the following text as the time constant or transmission time of the detection, as well as the magnitude of the threshold value.
A detector with a given time constant has the relationship shown in FIG. 2 between the signal-to-noise ratio SNR and the maximum actual transmission time Tac for a given reliability. By way of example, let us assume that a reliability value Pmc of 1% is required. For a given signal-to-noise ratio, that time period after the transmission of a signal is determined in which 99% of all the transmitted signals are detected in the receiver. The time period determined experimentally or theoretically in this way is the maximum actual transmission time Tac. The curve illustrated in FIG. 2 is obtained for a number of values of the signal-to-noise ratio SNR. Typically, the transmission time T0 for which the detector was designed is located in the region of a bend in the curve; if the signal-to-noise ratio is high, the maximum actual transmission time Tac does not fall significantly below T0 and, as the signal-to-noise ratio decreases, it rises very rapidly. The possibility of still receiving a command of restricted duration after T0 has elapsed is comparatively small.
If the signal-to-noise ratio is known, the time constant of the detector is made as short as possible while still allowing the required reliability to be achieved. However, if the signal-to-noise ratio is not known and it can vary over a wide range, then optimum detection is impossible. Either the transmission time is too long or the reliability is inadequate as a result of this.